Generally, an operational amplifier is a circuit to amplify a difference between input voltages. In the case of an ideal operational amplifier, transistors thereof have the same size and the same threshold voltage. Accordingly, when input voltages are equal, an output voltage from the operational amplifier must be 0V. However, the transistors do not have the same size and the same threshold voltage due to process factors. Meaning, there may be mismatch among the transistors. For this reason, even when the input voltages are equal, a voltage of several μV to several ten mV is output as an output voltage. This voltage is referred to as an “offset voltage.” Generally, this offset voltage is removed using a capacitor.
FIG. 1 is a circuit diagram illustrating a sample and hold circuit.
As illustrated in FIG. 1, the sample and hold circuit includes three switches S1, S2, and S3, one operational amplifier, and one capacitor Cc. Operation of the sample and hold circuit will be described as follows. When switches S1 and S3 are closed, input voltage Vin is charged in capacitor Cc. After a certain time, switches S1 and S3 are opened, and switch S2 is closed. In this state, a voltage obtained by adding an offset voltage to the sampled input voltage, is applied to the operational amplifier, as an actual input. As a result, the operational amplifier outputs a voltage obtained by eliminating the offset voltage from an output voltage, which contains the offset voltage.
In the above-mentioned configuration, it is possible to compensate for the offset voltage to some degree. However, it is necessary to use a non-overlap clock and a sampling capacitor. For this reason, it is difficult for this configuration to be used in a structure using a DC level driving buffer or a reference buffer included in an integrated circuit (IC). Noise may be induced in an IC due to continuous switching operation. In this case, operation of other circuits may be degraded. It is also necessary to increase the size of the capacitor used to reduce the influence of switches, etc. Furthermore, there is a difficulty in designing the operational amplifier because the operational amplifier should be maintained in a fixed state during an offset sampling operation thereof.